I wasn't using them at the same time.

Currently pay £20 a month (never bothered to switch and need unlimited use) and could do with a new router so makes sense to sign up to a new deal, the Sky phone line checker says I can get their Unlimited service which I think is the LLU one.

It's not a new phone line as such, a BT engineer just came round and fitted a new master socket and replaced the cable coming into the house from the pole outside. Not sure if that has affected contract but I thought I could keep a BT line with a LLU service?

Dead right .

"New line" doesn't usually mean the cable from the pole has been replaced, but let's move on.

Sky LLU Unlimited is arguably next to the best landline ADSLx broadband available, and you can't get the best. It is also cheap, with or without Sky TV. It does have traffic management and throttling though, so you would need to check the details out on the Sky forum on this site.

A "new line" would usually have a 12 or 18 month contract. A line repair does not.

Broadly speaking, I would go for it! It has to be better than what you have.

Not sure what your decision-taking level is in your household, but I suggest (without recommending for or against) you also consider the Sky Talk Calls bundle. I advise strongly against Sky Line Rental, as that puts you on to Full LLU, which often makes it a pain, possibly expensive and with loss of broadband for a week or so, to migrate to anyone else later.